A Comparison of Columnar-to-Equiaxed Transition Prediction Methods Using Simulation of the Growing Columnar Front
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THE columnar-to-equiaxed transition (CET) is a phenomenon that is sometimes revealed by macroetching a section of a cast component. The columnar region consists of elongated grains with a preferred growth direction. The equiaxed region consists of many grains with low aspect ratios and random orientations. A CET is formed at the shared boundaries at which the two zones meet. Sometimes the transition is more gradual and spread over a mixed columnar-equiaxed zone. There is a technical advantage to knowing if and when a CET may occur in a cast component. For example, directionally solidified turbine blades are manufactured to promote a columnar zone (thus avoiding the CET), because a columnar structure along the length of the turbine blade improves the creep resistance at high temperatures. In other applications, small equiaxed grains can give a higher yield strength and an improved liquid feeding in castings. Recently, Ares et al.[1] demonstrated how CET affects the resistance of a Zn-Al alloy to corrosion. They measured the charge-transfer resistance for a group of Zn-Al alloys. It was shown that, in some cases, the equiaxed zone could have better S. MCFADDEN, Postdoctoral Researcher, and D.J. BROWNE, Senior Lecturer, are with the School of Electrical, Electronic, and Mechanical Engineering, University College Dublin, Dublin 4, Ireland. Contact e-mail: [email protected] CH.-A. GANDIN, Senior Research Scientist, is with the CEMEF - UMR CNRS 7635, MINES ParisTech, Sophia Antipolis 06904, France. Manuscript submitted June 11, 2008. Article published online January 15, 2009 662—VOLUME 40A, MARCH 2009
corrosion resistance than the columnar zone. Thus, much attention has been given to the phenomenon of CET. Spittle[2] gave a recent review of CET experimental and modeling work. Columnar dendrites, which form the columnar grains, typically nucleate at a mold or chill surface. Initially, during a competitive growth phase, the dendrites grow under a moderately high temperature gradient. The preferred crystallographic direction for columnar dendrites is for h100i dendrite arms to grow in the direction opposite to the heat flow. In contrast, the equiaxed dendrites prefer to grow in the bulk undercooled liquid, where the temperature gradients are much lower. Equiaxed dendrites nucleate with seemingly no preferred crystallographic orientation. Equiaxed grains can originate in different ways. Hutt and StJohn[3] gave a summary of the origins of equiaxed grains. Typically, equiaxed grains nucleate at the site of an existing particle in the melt. Alternatively, equiaxed dendrites can originate from fragments of the columnar dendrites, and the detached arms grow to become equiaxed grains (Jackson et al.[4]). Regardless of the origin of the dendrites, it is clear that the condition of the bulk liquid determines whether a columnar zone or an equiaxed zone prevails. The growth restriction for a binary alloy is quantified in the literature as a factor Q,[5] where it is shown that Q helps establish the relationship between the underco
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